In the fabrication of beams and columns made of cast materials, a common approach is to cast a beam or column which cures to hardened concrete. Concrete is notorious in that it is relatively weak in tension but quite strong in compression. Performance of such a cast member is markedly improved by applying post tensioning, that is applying a compressive load on the cast member after curing. This changes the internal stress so that it is primarily in a compressive state along its full length. Several patents by Howlett set forth various and sundry tensioning devices, representative patents being U.S. Pat. Nos. 3,605,361 or 3,520,032. It is intended that such a tensioning device be placed permanently in a cast member to place a compressive load on the member for the life of the member. This can easily be 20 years, and would not unreasonably be 50 years. There is a measure of concern regarding long term failure modes. The failure mode typically is thought to be attack of the anchor plate, wedges and cable at the wedge. In the ordinary tensioned cast member, an elongate wire, typically a multi-strand woven wire rope, is anchored against a metal plate which bears against the end of the cast member. The wire is relatively small in diameter in comparison with the plate. The plate provides a relatively large bearing area to sustain the axial load applied to the member. Thus, compression is distributed by the anchor plate into the cast member. There is conjecture regarding various and sundry modes of failure. One conjectured mode of failure relates to electrolysis of the metal members. The electrolysis is aggravated by the locale where the tensioning member is located, thereby exposing it to rain and subsequent drying. During a year, the multiple cycles of drying and wetting as a result of rain may create undesirable electrical currents in the near vicinity. One protective approach is more aptly described in the patent of Schupack, U.S. Pat. No. 4,358,844. That disclosure seeks to place a completely surrounding electrical insulator about the anchor plate as a means of preventing the conjectured long term mode of failure primarily involving electrolysis.
The present apparatus is a very desirable protective member which is assembled at the time of fabrication of the post tensioned member to protect the anchor plate at the end of the cable. Thus, it cooperates with a conventional anchor plate which engages a multi-strand wire rope. The anchor plate, circular wedge, and woven cable and the tendon sleeve around the cable are not altered by the incorporation of the present apparatus. This apparatus can be used on appropriate sizing with anchor plates currently in vogue. Moreover, this apparatus is more aptly concerned with hermetic sealing to thereby prevent contact of the metal componets with chemicals or liquids which might be in the vicinity after installation. Corrosion protection is thus obtained by hermetic sealing so that the metal components are covered over and contact with oxygen is thereby minimized. The covering is achieved by constructing a surrounding housing or container which encapsulates the anchor and the other metal components. The housing is then filled, and indeed preferably overfilled with injected grease. The grease is of such viscosity to adhere to the surface continuously and to thereby provide lifetime hermetic sealing. Accordingly, the present apparatus is a structure to receive a charge of hermetic sealants, the preferred form being an oxygen repellent grease. Alternatively, the material can be mixed with an epoxy resin which cures to form a shaped encapsulation around the anchor plate and other metal components.
To have assurance of filling, the present encapsulating apparatus is constructed of parts which fit together very loosely. This assures that the device can be filled to expel most of the air. By visual observation, it is preferably filled and overfilled, the operator observing the excess flow of grease through the seams between the various components of the encapsulation structure. The encapsulation structure includes what might be termed a plastic tray sized and shaped to fit around the anchor plate. It is constructed with an elongate trumpet to extend around the tensioned wire rope and sleeve thereabout. It is conveniently sealed thereagainst with adhesive tape applied at the time of installation. Moreover, the tray is rectangular, fitting around the anchor plate. The anchor plate is then covered over by a mating nesting cover. The cover is sized to fit over the anchor and thereby define an internal cavity. The anchor cover in turn is open at the center. A large circular opening is provided. This exposes the metal components which are on the interior including the circular wedge which grips the cable and holds the cable in a tensioned mode. This access opening permits post tensioning of the cable after the concrete has cured. Moreover, a plug is incorporated which neatly fits within the circular opening left at the cover plate. The plug in turn is equipped with a grease fitting such as a zerk fitting. The zerk fitting is adapted to be connected with a hose fitting from a grease gun so that grease can be forced into the encapsulating structure of the present disclosure, filling the entire cavity with grease and overfilling so that grease is seen by the operator flowing out of the open seams between the structural components. In sum, the present apparatus is a three part construction, the components joining about the metal components of a post tensioning system. The encapsulating apparatus components are convenient to manufacture at low cost in view of the fact that they do not have to fit together snugly; indeed they preferably fit loosely to define surrounding open seams whereby an excess of injected grease flows out of the encapsulation structure. This assures that most oxygen on the inside has been expelled.